As for the high speed heavy duty cross ply tire, such as an aircraft tire, since the size and flight speed of aircraft have been increased recently, the operating speed of and the acting load on the tires have increased. Therefore, a tire is required to effectively lower the shock when an aircraft takes off or lands, and to have a resistance to a succession of large deformations, as large as 28 to 38%, for example, in the landing state.
As such tire for aircraft, tires having a cross ply structure, that is cross ply tire or bias tire, are widely used, where carcass cords are disposed to mutually cross between plies, since the tire having such structure has an advantage of having a high horizontal rigidity owing to the crossing of carcass cords.
And the ordinary bias tire for aircraft employs the nylon cord which is extendable in order to increase the bead durability as a carcass cord, to decrease the compressive stress F shown in FIG. 7, owing to the elongation accompanying the inflation of the tire. The compressive stress F is acts at the ends of turn up portion al of the carcass a around bead core b. And the repeated compressive stress is liable to cause the breakage of the carcass cord and premature rubber separation.
However, nylon cords are inferior in tensile strength, which results in the number of the carcass plies increasing such as at least 10 plies, ordinary more than 16 plies. As a result, the tire weight is inevitably increased, and fuel comsumption performance are decreased.
The present inventors discovered that the tire weight can be reduced by using newly structured aromatic polyamide fiber cords for the carcass, such cords having a very large tensile strength and a very high modulus like a steel cord, but the specific gravity is as light as nylon. The newly developed fiber cords have a limitation of the twist coefficient TN within a specified range.
The prior aromatic polyamide fiber cords are broken relatively easily when folded (when the tire is deformed largely), and the adhesion to rubber is inferior owing to its high degree of crystallization.